fix a bug in SkipListReader::skipTo().
fix a bug in SkipListReader::skipTo().

when the target is equal to a skip point, return the skip point prior to
the target, so that RTDiskPostingReader::decodeTo() could continue to
decode the doc freq value of the target doc.

izenecloud/izenelib,izenecloud/izenelib,pombredanne/izenelib,pombredanne/izenelib,pombredanne/izenelib,izenecloud/izenelib,pombredanne/izenelib,izenecloud/izenelib